Tetraethylorthosilicate (TEOS) and aluminium chloride were taken as sources of SiO2 and Al2O3 to prepare precursors of mullite by pre-hydrolysis of TEOS under refluxing conditions. Gelation was carried out at different pH values and the effect of the pH of gelation on the subsequent temperature-induced phase transformations was characterized by differential thermal analysis, powder X-ray diffraction and Si-29 and Al-27 solid-state nuclear magnetic resonance spectroscopy. A pH-dependent exothermic peak was observed at similar to 980 degrees C. Strong acidic conditions (pH = 1.5) were found to be beneficial for improving the mixing scale of the Al-Si components, leading to a mullitization temperature of similar to 1200 degrees C and a sharp 980 degrees C exothermic peak. In strong basic conditions (pH = 11.5), no evident similar to 980 degrees C exothermic peak was detected, and the mullitization temperature was as high as 1350 degrees C, probably due to heterogeneity in the mixing scale of the Al-Si components in the precursor system. A gel formation process has been proposed.